Overall reaction mechanism of photocatalytic CO2 reduction on a Re(i)-complex catalyst unit of a Ru(ii)–Re(i) supramolecular photocatalyst

Abstract

Rhenium(I) complexes fac-[Re^I(diimine)(CO)₃(L)]ⁿ⁺ are mostly used and evaluated as photocatalysts and catalysts in both photochemical and electrochemical systems for CO₂ reduction. However, the selective reduction mechanism of CO₂ to CO is unclear, although numerous mechanistic studies have been reported. A Ru(II)–Re(I) supramolecular photocatalyst with fac-[Re^I(diimine)(CO)₃{OC(O)OCH₂CH₂NR₂}] (R = C₂H₄OH) as a catalyst unit (RuC2Re) exhibits very high efficiency, selectivity, and durability of CO formation in photocatalytic CO₂ reduction reactions. In this work, the reaction mechanism of photocatalytic CO₂ reduction using RuC2Re is fully clarified. Time-resolved IR (TR-IR) measurements using rapid-scan FT-IR spectroscopy with laser flash photolysis verify the formation of RuC2Re(COOH) with a carboxylic acid unit, i.e., fac-[Re^I(diimine)(CO)₃(COOH)], in the photocatalytic reaction solution. Additionally, this important intermediate is detected in an actual photocatalytic reaction using steady state irradiation. Kinetics analysis of the TR-IR spectra and DFT calculations demonstrated the reaction mechanism of the conversion of the one-electron reduced species of RuC2Re with a fac-[Re^I(diimine˙⁻)(CO)₃{OC(O)OCH₂CH₂NR₂}]⁻ unit, which was produced via the photochemical reduction of RuC2Re by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), to RuC2Re(COOH). The kinetics of the recovery processes of the starting complex RuC2Re from RuC2Re(COOH) accompanying the release of CO and OH⁻ was also clarified. As a side reaction of RuC2Re(COOH), a long-lived carboxylate–ester complex with a fac-[Re^I(diimine)(CO)₃(COOC₂H₄NR₂)] unit, which was produced by the nucleophilic attack of TEOA to one of the carbonyl ligands of RuC2Re(CO) with a fac-[Re^I(diimine)(CO)₄]⁺ unit, was formed during the photocatalytic reaction. This complex works not only as a precursor in another minor CO formation process but also as an external photosensitiser that photochemically reduces the other complexes i.e., RuC2Re, RuC2Re(COOH), and the intermediate that is reductively converted to RuC2Re(COOH).